A method for employing a pre-crash sensor is known from a publication of the Fraunhofer Institute for Chemical Technology, published under the title "Airbag 2000" on the occasion of the Third International Symposium on advanced safety systems for vehicle occupants from Nov. 26 through 27, 1996 in Karlsruhe, Germany. It describes using a FMCW radar system. The system compares the amplitude of two harmonic components, which are selected by band-pass filters. Imminent collisions can be anticipated at a distance of about 1.5 m. The relative crash velocity is measured using multiple Doppler cycles to calculate the time remaining until a crash at a distance of 0.5 m. The object size can be included by using the difference between the amplitudes of the harmonic components and the distribution of the relative velocities. This method renders possible an early detection of an imminent collision between a motor vehicle and an object. For a more intelligent safety system actuation, however, additional information is often desired, particularly with respect to a so-called offset, i.e., a lateral offset between the motor vehicle and the object. The significance of this information is reflected in the fact that the crash behavior of motor vehicles in response to so-called offset crashes is increasingly becoming a standard used by the automotive industry.
U.S. Pat. No. 3,893,114 describes a method and a device for sensing a collision. A continuous-wave radar method employing a frequency-modulated wave is used. To analyze a reflected and re-received radar signal, the signal is initially mixed with the transmitted signal active at the time and, in a second step, with the modulation signal. A plurality of different mixed products result thereby, which also contain, inter alia, Doppler signal components due to the Doppler effect. On the basis of a comparison of instantaneous values of two selected mixed products, a time is defined, which is a measure of the vehicle's distance from and its velocity relative to an obstacle. As a function of this time, as well as of other conditions, a signal is generated, indicating that a collision is imminent.
Both related-art devices referred to above are suited for making a decision whether a collision between a vehicle and an object is imminent. However, neither of the two devices is able to determine a lateral offset between the motor vehicle and the object.